A Brushless DC (BLDC) motor refers to an apparatus for converting electrical energy to mechanical energy unlike a DC motor in the related art. While the DC motor in the related art operates with a brush corresponding to a temporary device having high abrasivity and a commutator (mechanical switching), the BLDC motor attracts attention as an apparatus for transferring and converting energy with high efficiency by using a semi-permanent power device (electrical switching) such as a high capability Insulated Gate Bipolar Transistor (IGBT) and a MOSFET. Due to such a reason, the BLDC motor is used throughout various industries relevant to an automobile and an industrial robot, and is expected to be continuously used in the future.
Electrical energy is transferred to the BLDC motor through an energy converter which is a driving apparatus. In general, the driving apparatus of the BLDC motor includes a power device (IGBT and MOSFET), a power device driving circuit (or gate-driver) and a motor driving circuit (or motor control unit). The power device serves to transfer a DC voltage to be supplied to the BLDC motor, and on-resistance, a breakdown voltage and a switching frequency are considered as being important in the power device. The power device driving circuit serves to turn on/off a power device gate so that the above-mentioned power device supplies DC power to the BLDC motor at a proper timing. Further, the power device driving circuit includes an over-current protection circuit for turning off the power device gate when a current flowing in the power device is equal to or larger than a reference current and a temperature protection circuit for blocking an operation of the power device gate when a temperature is equal to or higher than a reference temperature due to heat generated during a process of turning on/off the power device gate. Finally, the motor driving circuit includes a control block, a PWM generating block and a position estimating block. The control block is in a PI control form in order to reduce a difference of a control signal generated due to a difference between a command speed and a current speed, and should be optimized for effects such as responsibility improvement of the control unit and overshoot reduction. The PWM generating block is required for reflecting an output value of the control block to the power device driving circuit. In general, the output value of the control block is a voltage value or a current value reflecting capability of the control unit. The PWM generating block generates a signal in a PWM form to allow such values to be applied to the power device driving circuit. The position estimating block estimates a position of the rotor within the BLDC motor to determine a speed and the position of the current rotor, and generates a rotation signal of a next BLDC motor based on the determined speed and position. An optimized circuit design and algorithm of the above-mentioned blocks may influence the capability of the driving apparatus of the BLDC motor, but it is more important to understand how a mechanical system and an electrical system interwork with each other and to accurately detect the position of the rotor so that an electrical signal required for the mechanical system can be generated based on a movement of the rotor corresponding to an action of the mechanical system, which is properly transferred to the driving apparatus of the BLDC motor of the electrical system.
Prior document 1 (D fascicle of The Institute of Electrical Engineers of Japan, Vol. 107D, No. 5, pp. 628-634, 1987-5, Japan) discloses a method using a plurality of sensing coils to detect a movement of the rotor. An author of prior document 1 uses the plurality of sensing coils (eights coils) to spatially and temporally detect a variation in a magnetic field by considering a fundamental cause of generation of a rotation of the motor as spatial and temporal variations in the magnetic field of the rotor. Accordingly, the author describes that it is advantageous to detect the movement of the rotor in a transient state when there is a rapid change in loads. The prior document 1 includes the intention of making a motor operation technique more accurate by monitoring the variation in the magnetic field within the motor due to a hermeneutical technology and a methodological approach between the eight sensing coils. However, in prior document 1, it may be difficult to achieve mass production of the motor because of the complexity due to the use of the plurality of sensing coils, and there is a disadvantage in that a calculation unit for processing information on the plurality of sensing coils should be separately added to the motor driving apparatus. Further, prior document 1 has complexity in a system since each sensing coil has back electromotive force generated by the magnetic field of the rotor and thus a coupling effect generated between respective sensing coils also should be considered.
Prior patent 1 (US 2008/0084139 A1) discloses a method of installing a hall sensor in the outside of the motor to detect a position of the rotor through a sensing circuit. The motor is currently used in many industrial areas, and the method is one of several methods widely used in such industrial areas. The hall sensor detects a movement of electrons within the hall sensor changed according to a variation in a magnetic field by detecting a spatial variation of the magnetic field generated by a movement of the rotor. The hall sensor refers to a sensor using a “hall effect” in physics. In general, the hall sensor is frequently used in a servo motor system requiring an accurate movement rather than a cost reduction since the hall sensor has excellent performance but is expensive. Further, the hall sensor has a spatial limitation in that the hall sensor should be installed in the outside of the motor.
Prior patent 2 (US 2011/0175560 A1) discloses a method of indirectly estimating a position of the rotor by transferring voltage and current signals applied to the motor to various operation blocks within the motor driving circuit through a external current sensor interface. In prior patent 2, the operation block is inserted in the inside of the motor driving circuit in order to overcome spatial characteristics of prior patent 1 in which the hall sensor should be installed in the outside of the motor, and thus a position of the motor may be estimated without adding separate equipment to the motor. That is, prior patent 2 pays attention to the fact that a current applied to the motor is changed by the movement of the rotor and uses the current for estimating the position of the rotor. This means that a coil for driving the motor detects the spatial variation in the magnetic field by the movement of the rotor and a change in a current amount and a current direction by the detected signal is compensated for in a circuit, so that the position of the rotor is indirectly estimated. However, prior patent 2 requires various types of operation blocks for replacing a position estimating block of the rotor in the motor driving circuit such as a correction circuit due to a difference between interfaces for the voltage and the current of the motor driving circuit and an analog/digital circuit for a complex arithmetic operation by the current and the voltage and for implementing the arithmetic operation. For example, since a current waveform includes both a signal for driving the motor and a signal by the movement of the rotor, there is a need for a filter for separating the signals.
Prior patent 3 (US 2011/0175561 A1) discloses a method of estimating the position of the rotor using a structurally simple external single sensing coil unlike the hall sensor/position estimating circuit in a sensor/sensorless type in the related art. At this time, the external single sensing coil can be installed in a desired position and then used because the external single sensing coil is attachable and removable. However, in prior patent 3, the single sensing coil is arranged in the outside of the motor, so that a vibration of the motor may change a movement of the external single sensing coil. This corresponds to a fact that interruption must occur to estimate an accurate position of the rotor using the external sensing coil. In addition, measurement sensitivity of the external sensing coil may be influenced by a protection film in an outer cover type surrounding the motor.